1. Field of the Invention
The invention relates to circuit breaker circuit protection devices for electrical distribution systems. More particularly the present invention is directed to arc barriers within circuit breakers that may reduce potential erosion of moving contact arm and other internal circuit breaker components caused by electrical discharge arcing during circuit breaker contact separation.
2. Description of the Prior Art
Circuit breakers are utilized in electrical distribution systems to interrupt power current flow upon detection of a potential overload in the system. Generally circuit breakers are interposed in a power distribution circuit between a line source of power and a downstream circuit load. A circuit breaker commonly includes one or more fixed and moving separable contact pairs that open and close the power distribution circuit. A trip unit (often electromechanical, analog electronic or digital electronic) monitors circuit load and causes an operating mechanism to separate the contact pair (open the circuit) upon detection of an overload condition.
It is known in the art that during contact separation a current-induced arc of ionized plasma may form between the contact pair, potentially causing undesirable erosion of circuit breaker internal components, including the fixed and moving contacts as well as the moving contact arm. The electromagnetic properties of circuit breaker arcs can cause the arc to deflect toward the rotational axis of the moving contact arm during contact separation. It is desirable to shield the moving contact arm from such arc deflection.
Attempts to affix stationary shields to the circuit breaker housing would not protect the moving contact arm through its entire range of arm motion for all operational modes. Those skilled in the art appreciate that moving contact arms are often designed execute complex compound motion paths that are combinations of rotation and translation about an axis. Those complex compound motion paths often vary in different circuit breaker operating modes. By way of example, during manual operational mode, when a circuit breaker operating handle manually opens or closes the circuit breaker contacts, the operating mechanism is often designed to shift or translate the moving contact arm rotational axis along a path that intersects the motion path traversed by the breaker arm during a circuit fault interrupt contact separation that is initiated by the trip unit. Thus it is not possible to affix a stationary shield structure directly within the circuit breaker that would shield all ranges of circuit breaker arm motion: such a shield would block the circuit breaker moving contact arm motion path in one or more operational modes.
Moving contact arm shielding solutions attempted in the past have included translating the shield along the path of the contact arm in its various operational modes by (a) partially surrounding or fully enveloping the moving contact arm in a non-conductive material shield or (b) affixing sliding shields directly to the moving contact arm. Both of these prior solutions undesirably increase moving contact arm mass and potentially increase the bulk swept volume space occupied by the contact arm through its full range of motion. In attempted solution (a) the entire contact arm structure mass is increased and in attempted solution (b) the arm is forced to drag along the mass of the sliding shield. During a fault detection circuit breaker trip it is desirable to open and separate the contacts as quickly as possible in order to dissipate the arc. Any increase in contact arm inertial mass may undesirably slow contact separation speed. With respect in increase of contact arm bulk swept volume through its range of motion, there is a finite volume available within a circuit breaker housing to accommodate all components. A bulkier contact arm structure impacts surrounding and mating component potential occupied volume.
Thus, a need exists in the art for a circuit breaker apparatus that shields the moving contact arm from at least part of the arc created during contact separation through the full range of contact arm compound motion without increasing the arm's inertial mass and preferably not increasing its bulk swept volume.